Sujie Chen

Low-temperature MoO3 film from a facile synthetic route for an efficient anode interfacial layer in organic optoelectronic devices

A low temperature solution-processed MoO3 (sMoO3) thin film with a facile method for organic optoelectronic devices is developed. The film is extremely smooth with a root mean square (RMS) roughness of 0.318 nm, which is a significant advance for fabricating devices. An X-ray photoelectron spectroscopy (XPS) measurement shows that the sMoO3 possesses few Mo5+ states with a Mo : O stoichiometry of 2.99, demonstrating a nearly ideal MoO3 stoichiometry at a low annealing temperature. The sMoO3 film exhibits a superior hole injection ability in both an organic light-emitting diode (OLED) and organic photovoltaic cell (OPV), compared to conventional hole injection material poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). It is proved that the sMoO3 fabricated by the simple and low-cost method is an excellent alternative to a PEDOT:PSS anode buffer layer for solution-processed organic optoelectronic devices.

Large Area One-Step Facile Processing of Microstructured Elastomeric Dielectric Film for High Sensitivity and Durable Sensing over Wide Pressure Range

Once the requirement of sensitivity has been met, to enable a flexible pressure sensor technology to be widely adopted as an economic and convenient way for sensing diverse human body motions, critical factors need to be considered including low manufacturing cost, a large pressure detection range, and low power consumption. In this work, a facile approach is developed for one-step processing of a large area microstructured elastomer film with high density microfeatures of air voids, which can be seamlessly integrated into the process flow for fabricating flexible capacitive sensors. The fabricated sensors exhibit fast response and high sensitivity in the low pressure range to be able to detect very weak pressure down to 1 Pa and perform reliable wrist pulse monitoring. Compared to previous work, more advantageous features of this sensor are relatively high sensitivity being maintained in a wide pressure range up to 250 kPa and excellent durability under heavy load larger than 1 MPa, attributed to the formed dense air voids inside the film. A smart insole made with the sensor can accurately monitor the real-time walking or running behaviors and even a small weight change less than 1 kg under a heavy load of a 70 kg adult. For both application examples of wrist pulse monitoring and smart insole, the sensors are operated in a 3.3 V electronic system powered by a Li-ion battery, showing the potential for power-constrained wearable applications.

Inkjet printed fine silver electrodes for all-solution-processed low-voltage organic thin film transistors

Inkjet printed silver (IJP Ag) source/drain (S/D) and gate electrodes were incorporated into a bottom-gate bottom-contact organic thin film transistor (OTFT) architecture to develop all-solution-processed low voltage OTFTs. With well controlled ink wetting on a cross-linked polyvinyl-alcohol surface, IJP Ag S/D electrode pairs were fabricated with controllable short channels down to 20 μm using a Dimatix 2831 inkjet printer with a 10 pL cartridge, and formed good contacts with the organic semiconductor layer. IJP gate electrodes with a low flat surface profile were also achieved to obtain OTFTs of high quality gate dielectric layer for low leakage current. The fabricated low voltage all-solution-processed OTFTs present good device performance with a low operation voltage below 2 V, a mobility of 0.3 cm2 V−1 s−1, and an ON/OFF current ratio larger than 104.

High Sensitivity Flexible Capacitive Pressure Sensor Using Polydimethylsiloxane Elastomer Dielectric Layer Micro-Structured by 3-D Printed Mold

3-D printing is used to fabricate mold for micro-structuring the polydimethylsiloxane (PDMS) elastomeric dielectric layer in capacitive flexible pressure sensors. It is shown that, despite of the limited resolution of the used commercial 3-D printer for producing the mold, the fabricated sensor with the micro-structured PDMS layers can achieve sensitivity higher than previous work using micro-fabricated silicon wafer molds. The devices also present very low detection limit, fast response/recovery speed, excellent durability and good tolerance to variations of ambient temperature and humidity, which enables to reliably monitor weak human physiological signals in real time. As an application example, the flexible pressure sensor is integrated in a wearable system to monitor wrist pulse.

Annealing-Free Solution-Processed Silver Nanowire-Polymer Composite Transparent Electrodes and Flexible Device Applications

By using ultrahigh aspect ratio (>2000:1) silver nanowires (AgNWs) and ethanol-diluted poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the overcoating layer, we achieved flexible AgNW-polymer composite transparent electrodes of high conductivity and optical transmittance using facile solution processes at room temperature without annealing. The electrodes were applied in fabricating flexible capacitive pressure sensors and organic photovoltaic (OPV) devices. The pressure sensor with the composite electrodes presents three times higher sensitivity than that using ITO electrodes. A flexible 4 × 4 sensor array was also fabricated, which well proved the capability of the electrodes for spatially electronic signal collection and transmission. The fabricated flexible OPV device has a power conversion efficiency of 1.83%, which proves the potential of the electrodes for multilayer integration in optoelectronic device applications.

Low-Voltage pH Sensor Tag Based on All Solution Processed Organic Field-Effect Transistor

By reducing the sub-gap density of states at the channel, an all solution processed low-voltage organic field-effect transistor (OFET) was realized on a micrometer-thick SU8 gate dielectric layer. The device exhibits excellent operational stability under continuous bias stress. An integrated pH sensor tag was fabricated based on this low-voltage OFET, using an extended gate OFET for sensing and the other OFET as the load to convert the current signal to a voltage output for easy design of the subsequent readout circuit. With the low-voltage operation behavior and excellent operational stability of the OFET, the sensor tag was demonstrated to be operated in a 3.3 V battery powered electronic system for reliable pH sensing.

Highly Sensitive and Transparent Strain Sensor Based on Thin Elastomer Film

Strain sensors of high sensitivity and excellent transparency were fabricated based on a thin (~20 μm) poly(dimethylsiloxane) (PDMS) elastomer film integrating a conductive composite of silver nanowires and neutral-pH poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate). Such a thin PDMS film was peeled off from a glass substrate, which surface was modified with a self-assembled monolayer to reduce the required tensile stress in the elastomer film for the peeling-off process. The thin film sensor presented a sensitivity more than 20 times higher than that of strain sensors based on a thick (800 μm) PDMS film, and also a high optical transparency of nearly 80%, which are the desired features for developing unperceivable and invisible sensors in wearable applications. Moreover, the sensor showed excellent durability and fast response/recovery speed. An application for real-time monitoring of human neck posture was finally demonstrated.

Flexible Ammonia Sensor Based on PEDOT:PSS/Silver Nanowire Composite Film for Meat Freshness Monitoring

Flexible ammonia (NH3) sensors were fabricated on polyethylene terephthalate substrate using poly(3, 4-ethylene-dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)/silver nanowire (AgNW) composite film as the active layer. With the AgNWs of optimized concentration being incorporated into the PEDOT:PSS film, the sensitivity of the devices was significantly improved. Even with simple digitally dispensed parallel structure electrodes, the device achieved excellent sensing performance, and was shown to be able to detect very low NH3 concentration below 500 ppb. The mechanism for the sensing performance improvement was revealed. The sensor also showed considerable selectivity with respect to water and common organic vapors. Finally, the sensor was integrated with a self-designed portable data acquisition system to monitor the freshness of pork, demonstrating its feasibility for inspecting the meat quality in the early stage.

Silver nanowire/polymer composite soft conductive film fabricated by large-area compatible coating for flexible pressure sensor array

Soft conductive films composed of a silver nanowire (AgNW) network, a neutral-pH PEDOT:PSS over-coating layer and a polydimethylsiloxane (PDMS) elastomer substrate are fabricated by large area compatible coating processes. The neutral-pH PEDOT:PSS layer is shown to be able to significantly improve the conductivity, stretchability and air stability of the conductive films. The soft conductive films are patterned using a simple maskless patterning approach to fabricate an 8 × 8 flexible pressure sensor array. It is shown that such soft conductive films can help to improve the sensitivity and reduce the signal crosstalk over the pressure sensor array.

Silver Nanowire Mesh-Based Fuse Type Write-Once-Read-Many Memory

Fuse type write-once-read-many (WORM) memories are fabricated based on silver nanowire (AgNW) meshes using simple dispensing and bar-coating processes without needing precise patterning. Compared to previous work, the AgNW mesh WORM memories can be programmed at lower voltage (2.3 V) and current (<10 mA), and thus through near field communication by smart phones. The devices present stable “unwritten” and “written” states for reading operations of 6000 times. Finally, 4-bit WORM memories are fabricated and attached on a packaging box with programming and reading operations by a smart phone to verify its potential for practical applications.